Microlenses are used to focus light of a larger area onto a photodiode of a solid-state imager pixel, for example. Microlenses also can be used to trap light into solar cells. Also, light from a light-producing component can be transmitted through microlenses, for example, to project an image for display. Advanced products and systems that utilize microlenses in these and other similar ways include, without limitation, digital cameras, flat-panel visual displays, and solar panels. Such products and systems are used in a wide variety of practical applications.
The direction that light is propagated through two media, such as air and a lens, is based on the relationship between the refractive indices of the media. Snell's Law (Eq. 1) relates the indices of refraction n of the two media to the directions of propagation in terms of angles to the normal:
                                          n            1                                n            2                          =                              sin            ⁢                                                  ⁢                          θ              1                                            sin            ⁢                                                  ⁢                          θ              2                                                          (        1        )            The index of refraction (n) is defined as the speed of light in vacuum (c) divided by the speed of light in the medium (v), as represented by Eq. 2:
                    n        =                  c          v                                    (        2        )            The refractive index of a vacuum is 1.000. The refractive index of air is 1.000277. Representative materials used in microlens and semiconductor device fabrication include oxides, such as silicon dioxide (SiO2) with a refractive index of 1.45, and nitrides, such as silicon nitride (Si3N4) with a refractive index of 2.0.
When light travels from a medium with a low refractive index, such as air, to a medium with a high refractive index (the incident medium), such as silicon nitride, the angle of light with respect to the normal will increase. In addition, some light will be reflected. This will reduce the efficiency the imaging system, since not all of the light hitting the lens will travel through the lens to the photodiode, for example.
Light reflection that would occur at the interface between two media can be reduced if the two media have similar indices of refraction. U.S. Pat. No. 6,833,601 to Murakami teaches semiconductor imaging devices in which a refractive-index matching layer is provided over photodiodes of a solid-state imaging device. The refractive-index matching layer formed of mixed insulating-material compounds having a combined composition represented as SiOxNy. The oxygen and nitrogen contents of the layer are varied by regulating a mixture of insulating-materials during deposition. The mixture is regulated to have the lowest-oxygen/highest-nitrogen combined-content in the initial deposit, adjacent the photodiode-containing layer. The refractive index of this initially-deposited layer is similar to that of the photodiode-containing layer. As deposition continues, the mixture is adjusted to progressively-increase combined-oxygen content, and decrease combined-nitrogen content. Material having the highest-oxygen/lowest-nitrogen combined-content is deposited last, near the top of the refractive-index matching layer. As a result, reflectance at an interface between the photodiode and the refractive-index matching layer is reduced.
A microlens with reduced reflection of incident light would capture and transmit more light to the photosensor of a solid-state imager, for example. The increased light captured would include light that previously would have been reflected. Likewise, if the microlens were used in a display, reduced-reflection of the display light would produce a brighter display image.